This invention relates to a strainer or filter device (hereinafter referred to as "filter device") to be set in position in a fuel tank such as of an automobile and used for filtering a fuel being forwarded by a fuel pump to a carburetor.
Generally a filter device for disposition in a fuel line connected to a fuel pump or on a fuel pump within a fuel tank comprises an upper wall passing through and supporting in position the casing of the pump, a bottom wall spaced from the upper wall, and a cylindrical wall formed of a group of circumferentially spaced columns serving to support in position a mesh enclosing the space intervening between the upper wall and the bottom wall. Filter devices in which such walls are made of a plastic material have been disclosed by Japanese Utility Model Publication Nos. 37915/1977, 42840/1978, and 10917/1981. Ideally, the upper wall, bottom wall, and cylindrical wall would all be molded integrally in one piece of a plastic material. Since this is not realistic, Japanese Utility Model Publication Nos. 37915/1977 and 42840/1979 propose to form a cylindrical wall and a bottom wall integrally in one piece, cause engaging claws formed in the leading ends of the columns of the cylindrical wall to pass through engaging holes formed along the peripheral edge of an upper wall formed separately, and consequently secure the upper wall to the upper end of the cylindrical wall through the medium of these engaging claws. In addition to being complicated in construction, there is a possibility with this filter device that play may develop between the engaging claws and the upper wall fastened thereby because of imprecision in fabrication. When play is present, fuel which has escaped being passed through the mesh is forwarded by the pump and causes the carburetor to be clogged with foreign particles. Japanese Utility Model Publication No. 10917/1981 discloses nothing about the fixing of the upper wall or the bottom wall to the cylindrical wall. In any event, no other method has been available for constructing the filter device than by resorting to a complicated procedure such as is taught by Japanese Utility Model Publication No. 37915/1977.
Different plastic materials have different thermal expansion coefficients. In automobile fuels such as gasoline, methanol, ethanol and other alcohols, or mixtures of gasoline with such alcohols, molded plastic articles absorb these fuels and swell. The rate of this swelling likewise differs from one plastic to another. Owing to these variations, when different plastics are molded in the shape of doughnuts, for example, the articles molded of plastics having high coefficients of thermal expansion or high rates of swelling change greatly in outside and inside diameter and those having low coefficients of thermal expansion or low rates of swelling change only slightly in outside and inside diameter upon exposure to heat or fuels. The concept of coefficient of thermal expansion is well known and does not require further expansion. Therefore, only the rates of swelling will be discussed. The rate of swelling is as high as about 4% in the case of polyamide resins such as nylon and as low as about 1% in the case of polyacetal resins.